1. Field of the Invention
This invention relates to the field of helicopter tail rotors that produce aerodynamic thrust of variable magnitude to alter and stabilize the yaw position of the aircraft. More particularly, it pertains to pitch control actuation for such rotors enclosed within an aerodynamic control member forming a duct directed laterally with respect to the longitudinal aircraft axis.
2. Description of the Prior Art
A single rotor helicopter must have some means of compensating for main rotor torque. With a main rotor rotating counterclockwise, main rotor torque will cause the nose of the aircraft to turn opposite the direction of rotation, i.e. to the right. This condition is compensated by aerodynamic thrust produced by a tail rotor directed to the right producing a clockwise moment equal to main rotor torque. However, main rotor torque varies according to the magnitude of power applied to the main rotor; therefore, tail rotor thrust must vary to stabilize the yaw position and attitude of the aircraft as main rotor power changes.
The tail rotor is used also to control movement of the helicopter about its vertical axis. By overcompensating for main rotor torque, the nose turns leftward; by undercompensating, the nose turns rightward. Changing tail rotor thrust alters the magnitude of the yaw moment it applies to the fuselage so that the angular position of the aircraft with respect to the vertical axis is changed in accordance with control system input.
During operation in hover conditions and when flight speed is low, the helicopter is particularly susceptible to laterally directed wind gusts. This susceptibility requires frequent, precise corrective yaw control by the pilot to adjust tail rotor thrust.
The magnitude of tail rotor thrust varies with changes in the pitch or angle of attack of the tail rotor blades. Pitch links, or pitch control rods, connected to each pitch arm of the tail rotor hub at a position offset from the corresponding pitch axis, are raised and lowered concurrently, i.e. collectively, in accordance with control input to adjust blade pitch and tail rotor thrust as required to alter and stabilize the yaw position of the aircraft.
Conventional helicopter tail rotors are controlled manually by the pilot who manipulates pitch control pedals located in the cockpit. The pedals are connected by cables, bellcranks and push-pull rods to the collective pitch controls at the tail rotor. The tail rotor control system changes the angle of attack of the tail rotor blades and, in that way, the magnitude of the thrust force produced by the rotor. The thrust force produced by the tail rotor, directed laterally and located distant from the center of gravity of the aircraft, produces on the aircraft a yaw moment that continually reacts and compensates main rotor torque to maintain yaw position stability.
U.S. Pat. No. 4,809,931 describes a helicopter tail rotor enclosed within a circular duct located at the rear of a fuselage where a "V" empennage comprising two aerodynamic surfaces meets a tail cone extending from the cabin toward the tail. The rotor is mounted so that a component of thrust in the transverse direction produces a moment opposing drive torque of the main rotor and a component of thrust in the vertical axis produces positive lift. The two control surfaces of the empennage produce aerodynamic forces directed downward, one of the control surfaces producing a force having a horizontal component.
U.S. Pat. No. 4,585,391 describes a multiple blade tail rotor located in a duct directed transverse to the aircraft longitudinal axis. Fixed radially-directed blades, located inside the duct downstream from the rotor, recover energy of rotation from the airstream at the outlet of the rotor, thereby increasing thrust produced by the rotor.
German Patentschrift DE 29 26 180 C2 describes a rotor having blades grouped in radially opposite pairs, one blade of each pair extending radially from the rotor center, the spar or main structural member of each blade pair extending continuous across the rotor axis and formed integrally with the spar of the other blade of the pair. The spar is slotted so that the rotor axis is straddled by leading and trailing spar members. The slotted portion of the spar is long so that torsional displacement of the blade occurs over great length and at minimal stress in the spar.
Tail rotor control systems in the prior art operate to manipulate pitch links connected to each pitch arm and to a rotating control ring, which is raised and lowered along the rotor axis. As the control ring moves axially, the connection of the pitch links to the corresponding pitch arm causes each blade to rotate about its pitch axis, thereby changing the angle of attack and thrust produced by the rotor. A drive scissors is required to prevent rotation of an displaceable, non-rotating stationary control ring, which supports the rotating ring.
Ideally, the weight of tail rotors and their associated control systems should be light because they are located a considerable distance from the center of lift of the main rotor. This weight and its location are partially compensated in the prior art (as described in U.S. Pat. No. 4,809,931) by inclining the rotor slightly so that tail rotor thrust has a vertical, upward component.
Conventional tail rotors and their actuation and control systems are susceptible to shortened service life due to the inherent cyclic loading to which they are subjected and the adverse effect of this load environment on the metal components comprising these systems. Substantial care must be taken to prevent fretting of mating components, surface scratches and gouges and other such conditions having potential to reduce the fatigue life of the components. Due to the cyclic nature of the load environment, the service life of pitch bearings that support the blades and pitch shafts on each rotor arm must be kept at reasonable length to minimizes the operating cost of periodic replacement.